In general, a volume control is a potentiometer (variable resistor) for adjusting the loudness of an audio signal of a radio receiver or amplifying device. It is well known that a potentiometer ("pot") can be an electro-mechanical device which varies the loudness of a reproduced sound by varying the voltage amplitude of the electrically represented audio signal, by mechanically varying the resistance.
As a substitute for the mechanical potentiometer, electronic volume controls have been developed in the prior art. One electronic volume control uses an analog attenuator to attenuate the magnitude, or level, of sound in a continuous fashion, by varying the collector currents of transistors therein to change the internal resistances of the attenuator. In this analog arrangement, the controlling factor is an analog DC voltage amplitude.
In recent years, a digital volume control (DVC), or a "soft-pot" has been developed to vary the resistance digitally. In the DVC, the controlling factor is a digital word, having discrete values, which is software programmable to change the resistance. In communication devices, such as portable radios, including receivers, a DVC is often used in the audio path to provide programmable volume control of the audio amplifier. The audio path includes the DVC, along with a microprocessor, an analog to digital (A/D) converter circuit, the mechanical potentiometer (or volume control knob), and an audio amplifier. The A/D is included in the path to transform the mechanical volume setting to a digital setting for controlling the signal reaching the audio amplifier. This more complex volume control scheme serves to reduce the noise level in the demodulated audio picked-up when the audio path is routed to the mechanical pot.
However, in this audio path, a sudden sharp rise 12 or fall 14, set by the DVC, occurs, whenever a new digital word is programmed into the DVC, as seen in FIG. 1. The magnitude of the audio step depends on the difference between the old and new words, that is, the larger the difference, the larger the step. This quantization transient may also be amplified substantially by succeeding stages. For example, transients from changing the digital volume control (DVC) control word, are amplified by the audio power amplifier (PA). This commonly results in mechanical-like clicking sounds heard at the speaker which can be very irritating to the user. The magnitude of this problem is directly proportional to the magnitude of the change in the programmed DVC settings.
Even larger transients occur when the audio path is muted or unmuted as seen in FIG. 2. During priority scanning for the priority channel, for instance, the demodulated audio filtering path is rapidly muted and unmuted, resulting in "popping" noises at the speaker. Accordingly, there exists a need to reduce gain change and/or muting/unmuting signal transients.